On-board automatic tire inflation systems are currently available for use on trucks and other vehicles. Such systems automatically deliver make-up air to a vehicle tire in the event that the tire is punctured or begins to leak for other reasons. For a given trailer, truck, or other vehicle axle assembly, automatic inflation systems can be installed as appropriate for monitoring and maintaining a desired air pressure in each tire individually, in dual tire sets, or in the entire axle assembly as a whole.
By way of example, an on-board system currently available from Airgo, Inc. of Edmond, Okla. for maintaining a predetermined pressure in each of the tires of a truck tandem axle assembly comprises: a tire inflation system which delivers compressed air from an on-board compressor (e.g., the compressor used for supplying air to the truck brakes) to the interior of the tandem axle, or to a tube extending through the axle, in the event that a leak occurs in any of the four tandem axle tires; a pair of rotary seals provided proximate the outer ends of the tandem axle; a set of air lines extending from the rotary seals for delivering air from the interior of the axle, through the rotary seals, to each of the four tires; check valves provided in the air lines for preventing reverse air flow from the tires to the axle; and an indicator light which alerts the operator that a leak has developed. The system controls the make-up air flow in accordance with the operating pressure required by the tires. For most tandem truck axles, the automatic inflation system will preferably be operable for providing a sufficient rate of make-up air flow to maintain a tire pressure of at least 90 psig and more preferably at least 95 psig.
An on-board automatic inflation system of this type is described, for example, in U.S. Pat. No. 6,105,645, the entire disclosure of which is incorporated herein by reference.
A need currently exists for an economical gas flowmeter which can be used in on-board automatic tire inflation systems for determining leakage severity by accurately measuring the rate of make-up air flow delivered by the inflation system. In the event of a nail puncture wherein the nail remains in the tire, the resulting air leakage rate from the tire can be as low as 0.1 milliliter per minute (“ml/min”) and will typically be in the range of from about 0.1 to about 40 ml/min. Air leakage rates of up to 100 ml/min or higher can result if the nail is removed or if other types of punctures occur.
As will be understood by those in the art, conventional thermal loss devices, capillary pressure loss devices, and positive displacement devices capable of accurately measuring flow rates of as low as 0.1 ml per minute typically are expensive and are not suited to handle the moisture and contaminant levels present in on-board compressed air systems. Some devices also are not suited to handle the vibration levels encountered during vehicle operation. In addition, the on-board air compression systems in many vehicles are not sufficient to accommodate the amount of pressure loss produced by the operation of some conventional flowmeters such as positive displacement devices.
Flowmeters which either employ a flow element having a single orifice or employ a flow element having a collection of multiple large orifices or pores such as disclosed in U.S. Pat. No. 4,040,293, U.S. Pat. No. 5,461,932, U.S. Pat. No. 5,722,417, or U.S. Pat. No. 6,164,142 also are not well suited for use in automatic tire inflation systems. These devices are susceptible to plugging and, even when the flow element has a collection of orifices extending therethrough, the device can cease to operate effectively if a few or sometimes even if just one of the orifices is/are blocked.
U.S. Pat. No. 5,357,972 discloses a disposable pneumotachograph flowmeter for measuring human respiratory flow. The pneumotachograph flowmeter comprises a pair of conical mouth pieces having a fabric screen made of a hydrophobic fabric material sandwiched therebetween. It is stated that the hydrophobic fabric material prevents the fabric screen from being occluded with moisture. The fabric screen is said to have about 400 pores per square inch with an average pore size of 2.25×10−6 square inches so that laminar flow through the screen is enhanced to provide a more linear relationship between flow and pressure drop.
The only use disclosed or suggested in U.S. Pat. No. 5,357,972 for the disposable pneumotachograph flowmeter is for measuring respiratory flow. Moreover, neither the disclosed disposable pneumotachograph flowmeter nor the fabric screen employed therein is suitable for use in an on-board automatic tire inflation system. Although the fabric screen of the pneumotachograph flowmeter is hydrophobic, the fabric will likely absorb other contaminants such as oil droplets which are typically present in truck air systems. In addition, a fabric screen is not well suited to handle the vibration levels encountered in on-board tire inflation systems and, because a fabric screen has little or no inherent stiffness, significant additional measures would be required to contain and hold the fabric screen in operating position and to maintain its shape.